How do you get into the tunnel in Mordred's lab on Astro Knights Island?

Answer 1

Push on the wall at the left side. It is only dirt and it will fall down. The tunnel leads to the back of the dungeon, so you will need to have thrown the switch from the Library entrance, and you will need to feed the mechanical mouse to Mordred's owl.

To enter the underground lab entrance (between the two hay bales on the farm):

1. Get the secret message from the Maid's room (castle first floor, to the left)
2. Take it to the girl at the mill (upstairs) to get the Cosmic Symbols password
3. Go to the fountain, on the brass plaque. Press the 3 symbols and the sun.
4. Get the key from the hooded fellow at far right.